Fix wrong file name when --output option is used

Nazar Kalinowskirequested to merge
nazark/spectacle:bug424380 into master

BUG: 424380

After dec0e6cef23ac35754aa537db6d7ace42b23ca2a the "--output filename" option was totally ignored, this patch fixes it.

Edited by Nazar Kalinowski

Merge request reports